1. Field of the Invention
The present invention relates to a base station frequency assigning system for a mobile communication system, and more particularly to a frequency assigning system for each of base stations to automatically assign a frequency to be used for the base station.
2. Description of the Related Art
In a mobile communication system, frequencies or radio channels to be used by a base station are assigned to each base station. A frequency assigned to a base station is also assigned to other base stations which are physically located far away from each other and no interference of frequency occurs between the base stations so as to use a limited frequency effectively.
Even if a frequency to be used for each base station has been assigned in advance in this way, when a new base station is installed additionally due to a service area expansion or new radio channels are additionally introduced due to expansion of traffic capacity later on, it is necessary to change an existing frequency assignment and a frequency assignment should be performed again. One of conventional method of such a frequency assignment is performed by measuring and computing an amount of interference among frequencies used by the base stations and new frequencies to be used are decided. However, it needs a large amount of computational effort.
Thereupon, a Japanese laid-open publication Tokkaihei No. 4-91522 has proposed a frequency assigning system for automatically assigning frequency to be used for each of base stations. In this system at least one station of plural base stations has a means for monitoring frequencies of control channels used by its surrounding base stations. And according to output of this monitoring means, the base station determines a frequency for its own control channel.
In this proposal, a base station detects frequencies used in its surrounding area by the monitoring means, and assigns to itself a frequency being not used in its surrounding area or a frequency, even being used by other base station, which does not cause interference due to weak signal receiving level of the frequency. Therefore, each base station itself can perform a frequency reassignment.
This base station may also be provided with a means for monitoring a frequency of control channel of a mobile station being in its neighboring radio zone whose base station is blocked by a large building or the like. Therefore, even if a frequency radiated by the neighboring base station does not reach to the base station being assigning frequency, the base station does not select the same frequency being used by the neighboring base station.
In the base station frequency assigning system according to this proposal, since a base station determines its operational frequencies one by one, there has seen a problem that it takes a considerably long time to determine operational frequencies of all base stations. And although the proposed technology might not select the same frequency as used in the neighboring base station by monitoring a frequency of control channel of a mobile station being in its neighboring radio zone whose base station is blocked by a large building or the like, it is not perfect to avoid selecting the same frequency because there is no guarantee that all frequencies being used in the neighboring base station can be monitored.